Parallel vs Series

[SlimTim]

I should know this but I never gotten a definite answer.
When two water heaters are better than one is there ever any benefit in connecting them in parallel? I always do it in series with bypasses in case one of them leaks. It seems like the most efficient way

I know the first one will get the brunt of the work but no more than if it were in there by itself. I would think in parallel you might get a mix of cold water if one of them depletes sooner than the other for whatever reason.

[JCsPlumbing]

The manufacturers schematics recommend in parallel.

Instead of using anode rods your supposed to use River Birch tree roots.:)

[SlimTim]

I tried that but the water smelled like sour milk!

[JCsPlumbing]

Kidding with ya'. But honestly I have schematics around here somewhere that recommends in parallel. There reasoning, if I recall correctly, is the heaters wear evenly with the same performance. The other way, the inlet heater wears faster. I've thought about this and couldn't calculate if this is a true benefit or not. (Parallel) 2 spring a leak in 10 years or 1 springs a leak in 1.

J.C.

[JCsPlumbing]

I meant to say 2 spring a leak in 10 years or 1 springs a leak in 8 maybe.

[SlimTim]

Which makes me think "ok, I would rather have one go and then another a few years later rather than both go at once."

[ToUtahNow]

In series you are preheating the water in the first tank and it is often a smaller heater (less BTUs) and/or set at a lower temperature than the second heater. In parallel you are increasing your capacity by using two heaters of the same size and the same BTU rating.

Mark

[JCsPlumbing]

Utah, how does that calculate out as a benefit. Seriously need to know.

Let's say I size a project that needs 2-50 gal. 50,000 btu heaters in parallel to give adequate performance. What would be the "Series" equivalent? And why would the parallel or series be a better choice? Thanks.

J.C.

[SlimTim]

And what would prevent one of the tanks from depleting first and mixing cold with the other?

[JCsPlumbing]

Slim, when I've done parallel, I always try to keep the inlet piping to exact measurements on both heaters. So, same cold source, same measurements from the stubout everywhere to each one and coming from the hotsides to the hot stubout.

[SlimTim]

Is that always possible? I mean, that cold supply is never coming up exactly between those tanks.
I may be simple-minded (indeed I am) but when i put two 50's in series I know I've got about 94 gal of hot water, no question about it.

And as long as we're talking about hot water...These on demand tanks that are getting so popular, in Memphis the water coming from the city in the winter is below 40 degrees. The best I've seen these tanks do is somewhere around 7 gpm at 90 degree rise. Does that sound right. I need some input on these as I am asked about them every week

[ToUtahNow]

Without doing too much math, if you have two 50 gallon water heaters at 50,00 BTU with each having a first hour delivery of 83-gallons. Assuming both have the thermostats set the same you will have 166-gallons of first hour delivery. This would work well if you had a large soaking garden tub.

In series one water heater is a kicker for the other and does not run all of the time (hence different temperature settings). You will not have as much capacity but you won't have the fuel costs either.

Mark

[JCsPlumbing]

Thanks Utah, I kinda' thought that but you also taught me something.:)

Slim, if I'm roughing it in, it IS always possible. If you are going to replace you can recenter the stubs to equalize flow.

As far as tankless, I pretty much discourage them. I get asked as much as you do. I usually prove mathematically that you will lose money with one. People that have or want them will argue with you 'til they're blue in the face so I'm almost to the point of just telling them "you're right."

I just attended a Rheem/Paloma seminar and they are showing numbers that state tankless may be the future based on sales numbers but were strict to state that they were not the best use for every application.

I personally think the Rinnai is the best currently but that could change. The real mess I see for the future is that Rheem/Paloma, Rinnai, Takagi etc. all have different parts as opposed to alot of the traditional ones that we can find things for most everywhere. People are buying these things everywhere including the internet. So when this one malfunctions or that one malfunctions, whose got the part? Supply houses can't keep a control board, thermistor, etc. for all of them. Instead of being able to get people back up and running alot of times in 1 hour. It could be a minimum of 1 or 2 days.

And pay close attention to groundwater temps in winter. That's what I use to size hotwater for houses/businesses.

[SlimTim]

Thanks jc, I did recently have someone at the supply house say they wished they could find someone to do the warranty work on them (ruud).

And, if the people think they have "endless hot water" won't they tend to use more?

[JCsPlumbing]

Yes, which factors into the cost/payback equation. Rheem/Ruud/Paloma-same company. All owned in Japan (I think) by the Paloma group.

One final note. I don't trust the degree tables any tankless company supplies. I measured the temps. in the winter at different places in my area vs. rise required and concluded I would probably see more problems, callbacks, and unhappy customers.

And I don't like that!

J.C.

[Bogart]

You get more hot water with a series installation. The rule of thumb is that you get 75% of the heater's capacity as hot water, before the incoming cold has cooled off the water. Two parallel 50's give you 75 gallons of hot water at one shot. Two series 50's gives you 87 gallons at one shot. Most parallel installations end up favoring one over the other, and you have a heater sitting there.

[westcoastplumber]

I am a huge supporter of Rinnai, they are the best tankless on the market.

Tankless is not for everyone, depends on the customers needs.

They also have to be installed correctly, few are.

[aero1]

I am a huge supporter of Rinnai, they are the best tankless on the market.

Tankless is not for everyone, depends on the customers needs.

They also have to be installed correctly, few are.
i agree.:D

[mtnman1100]

In series, if the second goes down then you better have bypasses installed.
Don't you guys see two tank installs mostly in commercial? I do. The amount of space it takes for 2 50's in a residence is usually better served by a 75 gallon tank.